Realistic TRC1001 Problem

[absorbentgnome]

Hello everyone!

I have a Realistic TRC-1001 handheld cb transceiver that is annoying me. It receives but does not transmit - not even a whisper.

It has a LC7137 PLL and MC3357P reciever, a 2SC2075 final and a 2SC2314 driver. I have checked the final and it's driver by removing them and testing the forward junction voltage drop and they both checked OK. I also checked some 2SC1815 transistors nearby and they were OK too.

At first I thought that perhaps the 10.24Mhz PLL crystal had failed, but it needs the PLL to recieve?

So out came the oscilloscope and it was attached to some of the test points available. There was oscillation on test points near the PLL inside a metal can. When transmit was pressed, all oscillation stopped. No oscillation could be detected in any of the driver and final stages.
The signal to transmit does appear to be present at the PLL, one of the test points went from high to low upon pressing transmit.

Why would oscillation stop when TX is pressed?

Any help on this would be greatly appreciated

[Rick]

Check TR12 and 13, what is the TX and RX frequency at TP4 which is the emitter of TR15?

Rick.

[absorbentgnome]

Thanks Rick for your help!

What appears to be Q12 (and Q13 ) are in the metal can at the top right of it. The other two, beneath them are covered in wax. Also, the back of the PCB has a screening plate soldered to it. What is the best method of removing this plate?

Will have a go at removing it and will post back the results of Q12 and Q13. I do not have any equipment to measure the frequency at TP4, which must be the point at the bottom right of the can. All I can check for is some form of oscillation using my old 20Mhz scope.

Thanks again for your help

[absorbentgnome]

lol after getting very annoyed i got the @@%%$ plate off ok

[absorbentgnome]

Ok tested Q12, Q13, Q14 and Q15 and they're ok. Q13 stood out from a different forward voltage but it turned out to be a different transistor, a 2SC1973 instead of a 2SC1815.

TP4 on recieve measures 2.15V dc on a multimeter. On the scope, it measures 2.1V dc with a small signal present. Switching to AC to remove the offset reveals a 160mV pk-to-pk high frequency waveform that my scope cannot resolve.

Upon transmit, TP4 instantly falls to 0V with no small signal present.

[absorbentgnome]

The other test point in the can has 1.6V dc on it on recieve and 0V on transmit.

[absorbentgnome]

I managed to get the tip of my probe onto pin 18 of the LC7137 and it appears that VDD, which is 6V on recieve drops to 0V on transmit, which clearly is the problem.

What could cause this? Following the tracks on this is impossible!

[absorbentgnome]

Wondering where the supply for the PLL came from, I spotted two 78L007 regulators at the bottom of the unit. One (IC5) has 12V at the input, 0V at the common and 7V at the output. The other, IC1 has 7V at the input, 0V at the common and 5.5V at the output. Pressing transmit causes the supply to drop to 0V as well as the input to the regulator to 0V. Clearly something is wrong here sice it should be at 7V all the time. However, the input to the regulator isn't directly connected to 12V like the other one. Trying to work out exactly how it is configured.

[absorbentgnome]

There was a 220ohm resistor in series with the regulator, I replaced with a 100ohm to raise the voltage and got a nice 7V at the regulator output. This also improved the receive performance.

However, there still is a short somewhere in the TX/PLL circuit that is shorting out the 7V supply from IC1.

[absorbentgnome]

Got the bastard! To save power, the power to the Mic amplifier with AGC (UPC1170H) was switched. The IC was short.

I finally narrowed it down by removing the TX signal to the PLL to ensure nothing changing in the PLL circuit was faulty. Keying up resulted in a short, so my attention went away from the PLL - and the only thing left was the mic amplifier. Desoldered it and wahey back came the TX power (but obviously no modulation).

Talk about a round-about way to find the fault!!!! This has taken me ALL day.

Now i've gotta find somewhere that will sell me a UPC1170H.

Thanks for your help earlier Rick

[absorbentgnome]

New chip arrived and the handheld now works fine. Good modulation and when used with an external aerial, as sensitive as my main rig.

[mojo73]

Hehe, Congratulations on beating the sucker!


That was a good read actually. Well done.

[absorbentgnome]

Thanks, it was a pain in the arse. Stupid thing was if I had checked the regulator voltage under TX then it might not have taken me so long. Went on a wild goose chase all around the TX circuitry trying to find shorted transistors. Annoying, particularly with all the inductors (and the metal plates)